Using a front handlebar bag on a randonneur style bike, a front rack supports the bag ok, but unless the bag is secured somehow at the top, it will flop around quite a bit. A "decaleur" is a rack support made specifically for this. They are sometimes tricky to set up and then you're stuck with it when you're riding without a bag.
Looking through and old VAR tool "Petit Livre Jaune" I noticed an illustration of a VAR lightweight decaleur support that loops over the bars. Inspired by this design, I made my own by bending a section of 5/16" aluminum round. I started out with a piece of a wire coat hanger to get the basic bend shape figured out and then used a tubing bender for the final shaping with the aluminum round. I filed and sanded the hook ends that fit into the opening in the back of the leather reinforcement strip of my handlebar bag, a Berthoud 2286.
I used handlebar tape for padding where the decaleur hangs off the handlebars. This mini decaleur supports the bag fairly good, strapping down the front of the bag to the rack with a piece of nylon cord makes this set-up excellent. When not riding with a bag this decaleur is easy to remove and store in the bag until it's needed.